1. Field of the Invention
This invention relates, in general, to radar systems and, more specifically, to airborne radar systems which detect moving targets.
2. Description of the Prior Art
Airborne radar systems are always susceptible to the reception of ground clutter signals when scanning the ground for radar targets. The ground clutter return signals are reflections from the ground and various objects in the target area which are not regarded as targets. The conventional way to detect moving targets is to detect a return signal which has a frequency which has been shifted from the originally transmitted radar signal frequency. The shift in radar frequency occurs due to the Doppler effect caused by the movement of the target.
In addition to the Doppler effect caused by the movement of the target, shifts in frequency also occur due to the movement of the aircraft containing the radar components. When the frequency shift due to the motion of the target is sufficient, the radar apparatus can detect this frequency shift against the background of other signals returned from the target area. Thus, a moving target can be detected. However, with conventional radar systems, the speed of the moving target must be relatively high in order for the radar system to detect a sufficient amount of Doppler shift to recognize the target.
In a typical monostatic radar system in which the radar transmitter and receiver are located aboard the same aircraft, the speed of a moving target on the ground is often not great enough to produce a shift which can be recognized by the radar system. Detecting moving targets, such as other aircraft, is not such a difficult problem due to the relatively high speed of the target. However, slowly moving targets on the ground, such as trucks and tanks, are difficult to detect from airborne radar systems.
The limiting factor involved in the detection of moving targets from airborne radar systems is the spread or width of the frequencies existing in the ground clutter or return clutter from the target area. With conventional monostatic radar systems, the amount of ground clutter spread existing in the frequency spectrum of the received radar signal is dependent upon the speed of the aircraft with respect to the target area, and is also dependent upon other factors. With a wide spread in frequency, it is difficult to detect a Doppler shift which is small since the frequency received by the radar system receiver is masked by the ground clutter signals. Therefore, to sufficiently detect slowly moving targets within the target area, it is necessary to reduce the amount of Doppler shift spread received due to the ground clutter return.
The spread of the ground clutter is dependent upon the beamwidth of the antenna, since the smaller the area on the ground whhich is illuminated by the radar transmitter, the smaller will be the frequency extremes from leading to trailing edges of the radar beam produced by the ground clutter. Therefore, a narrow beam-width is helpful in reducing the amount of ground clutter spread in the received radar signal. However, narrow beamwidth cannot be obtained easily without increasing the size of the radar antenna or by increasing the frequency of the transmitted radar signals. Increasing the size of the radar antenna is seldom practical in airborne radar systems. Using higher frequencies is technically feasible but presents other problems, such as excessive attenuation by the atmosphere and excessive back scattering from rain. Therefore, the advantages gained by increasing the frequency to produce a narrower beamwidth are to a large extent counteracted by the distances of using a higher radar frequency.
Other techniques may be used to reduce the amount of return signal spread from the ground clutter, such as by the use of displaced phase center antennas. U.S. Pat. No. 3,438,030, issued on Apr. 8, 1969, describes such a system. However with such systems, it is important that the antenna sections give precisely the same performance, thus construction and adjustment of such an antenna system is critical for proper operation. In addition, a displaced phase center antenna radar system can only use a few number of pulses in the transmitter radar signal instead of a large number of radar pulses. As is well known to those skilled in the radar art, using many pulses in the radar signal permits easier filtering of the return signal at the radar receiver.
Therefore, it is desirable, and it is an object of this invention, to provide a radar system for detecting moving targets which does not require displaced phase center antenna arrangements and does not require the use of a higher transmitting frequency. It is an object of this invention to provide a radar system which is capable of detecting slowly moving objects when the radar system is located aboard relatively fast moving aircraft platforms.